A well-known electrophotographic image forming apparatus has a fixing device that thermally fixes a developer image to a sheet. There have been proposed methods for controlling a temperature of the fixing device. According to one of such methods, a temperature of a back-up roller of the fixing device is detected at certain intervals once a series of printing is initiated after a period of stand-by, and a temperature of a heat roller is controlled to decline in a phased manner each time the detected temperature reaches a predetermined temperature.